The present invention relates generally to a low noise voltage controlled oscillator (VCO), and more particularly to VCOs employing an LC resonant circuit having high capacitance and low inductance in combination with amplifiers and impedance transformation networks to provide a low phase noise RF signal.
Electrically controlled or voltage controlled oscillators are used in many applications, one example being indirect frequency synthesis utilizing phase-locked loops. An effective way to achieve electrical tuning in VCOs is to use varactor diodes to provide a variable capacitive element in a LC resonant circuit. The varactor diode has a variable capacitance that is dependent upon the reverse bias applied across the diode. Varying the reverse bias voltage applied across the diode varies the capacitance hence tuning the oscillator. While varactor diodes can tune over a wide frequency band while exhibiting low loss characteristics comparable to mechanical variable capacitors, varactor diodes are non-linear capacitors and exhibit noise mechanisms nonexistent in conventional capacitors. Varactor diode tuned oscillators typically exhibit higher phase noise than those employing mechanical tuning. This effect is especially pronounced when the VCO is tuned over a wide frequency band, such as over 10 percent in terms of band ratio. One prior art solution to this problem has been to use several narrow bands to cover a desired frequency band by switching in additional fixed capacitance to change from one band to the next or by providing two or more narrow band VCOs.
Effects inherent in varactor diodes are the dominant factors contributing to a high phase noise level in the wide band VCOs. Since the varactor diode is a non-linear device, a large high frequency (RF) voltage results in a non-linear capacitance variation that is a function of the RF voltage. This non-linear capacitance variation produces a higher phase noise level in wide band VCOs. U.S. Pat. No. 4,536,724 entitled "Voltage Controlled Oscillator Having Three or More Varactor Diodes", issued on Aug. 20, 1985 to Makoto Hasegawa et al discloses a VCO having an LC resonant circuit including a varactor circuit comprising three or more varactor diodes connected in series so that the RF voltage applied to the varactor circuit is divided across the series connected varactor diodes thereby minimizing the RF voltage applied across a single varactor diode. U.S. Pat. No. 4,621,205 entitled "Method and Apparatus for Reducing Varactor Noise" issued on Nov. 4, 1986 to Brian M. Miller discloses connecting blocks of parallel connected varactor diodes in series to reduce varactor diode noise.